Sheathed electric heater insulating material



Feb. 16, 1954 w. T. RAWLES 2,669,636

SHEIATHED ELECTRIC HEATER INSULATING MATERIAL Filed May 21, 1951Inventor: Wi I iam T Rawle s,

bps m H i s Attorne g.

Patented Feb. 16, 1954 SHEATHED ELECTRIC HEATER INSULATING MATERIALWilliam T. Rawles, Pittsileld, Mass assignor to General ElectricCompany, a corporation of New York Application May 21, 1951, Serial No.227,384

Claims.

This invention relates to sheathed electrical heaters and particularlyconcerned with an improved insulating material for such heaters. It hasas its principal object the provision of a heater of this characterincluding an improved heat-refractory insulating material.

The sheathed heaters of the type with which the present invention isconcerned are those in which a resistance conductor is enclosed in atubular metallic sheath, and embedded in and supported in spacedrelation with the sheath by a densely compacted layer of heat-refractoryand heat-conducting, electrically insulating material.

Such a heater is shown in the accompanying drawing in which the singlefigure is a fragmentary view in elevation of a sheathed electric heaterembodying the present invention, portions of the heater being shown insections so as to illustrate certain details of construction.

As shown in the drawings, the heater comprises a helical resistanceconductor 19 mounted within and substantially centrally of a tubularmetallic sheath H. The resistance conductor Ill may be formed of anysuitable material, such as a nickel-chromium alloy. The sheath I lpreferably is made of a suitable chrome-iron-nlckel alloy. Theresistance conductor It is secured at its ends to suitable terminals [3.These terminals may be formed of any suitable material, but preferablywill be formed of steel. It is to be understood, however, that theconductor in, the sheath I! and terminals 13 may be made of many othersuitable materials.

The resistance conductor Ill is embedded in and is supported in spacedrelationship with reference to the sheath by a mass 12 of heatrefractory and heat-conducting, electrically insulating material. Themass of insulating material is in granular form and is compacted to ahard dense mass in any suitable manner, as by reducing the diameter ofthe sheath H after the resistance conductor, terminals, and insulatingmaterial have been assembled within the sheath. This reducing operationmay be accomplished by swaging, drawing, .or rolling.

The heat refractory insulating material that has found wide applicationin such sheathed electric heaters is granular magnesium oxide, andheaters provided with this insulating material have on the whole beenquite satisfactory. Pure magnesium oxide has a very high initialresistance and maintains a high resistance; value throughout a very longlife. The granular magnesium oxide is prepared by fusing magnesium oxideand crushing and pulverizing the fused product to the desired granularsize.

Due to the fact that pure magnesium oxide is not available commerciallyin suflicient quantitles or at a reasonable enough cost, commercialmagnesium oxides have ordinarily been employed after first being treatedor modified to obviate the undesirable effects of the impuritiesordinarily present in such commercial magnesium oxides. These treatmentshave included the incorporation into the fused magnesium oxide of asmall amount of silicon oxide for the purpose of counteracting the smallquantities of calcium oxide present as impurities in commerciallyavailable magnesium oxides or the treatment of the fuse magnesium oxidewith beryllium oxide by the process and for the purposes described inPatent 2,170,692, issued to Louis Navias, and assigned to the sameassignee as the present invention. However, even these modifiedcommercial magnesium oxides have not been completely satisfactory fromthe cost standpoint.

Various attempts have heretofore been made to substitute for magnesiumoxide less expensive materials such, for example, as zirconium silicatewhich is obtainable in substantial quantities at a lower cost. However,the extreme hardness of granular zirconium silicate and the crystalstructure thereof do not permit this material to densify easily whenpressed, rolled or swaged in the manufacture of the heating element.Granular zirconium silicate has been found to flow to such an extentduring the reduction of the sheath that the resistance conductor doesnot remain centered within the sheath during the reduction operation,Also as the zirconium silicate is not sufficiently densified orcompacted, shifting of the helical resistance conductor within thesheath was frequently encountered during use of the heater element.

The present invention is based on the discovcry that a low costrefractory insulating material possessing satisfactory electricallyinsulating and heat-conducting properties and the proper compactingcharacteristics can be provided in the form of mixtures of zirconiumsilicate and magnesium oxide of particular particle sizes. The term"magnesium oxide as used hereinafter is intended to include puremagnesium oxide as well as the modified commercial grades of magnesiumoxide, all of which have been prepared by fusing and thereafter crushingand pulverizing the fused material tothe propergranular size.

More particularly, it has been found that mixtures of from about to percent by weight of granular fused magnesium oxide and from to 80 per centzirconium silicate can be satisfactorily employed in the manufacture ofthe above described sheathed electric heating elements provided theparticle size of the magnesium oxide is such that at least about 22 percent is retained on a (SO-mesh screen while all of it passes through a40-mesh screen and the zirconium silicate is of a particle size lessthan 80 mesh with at least 80 per cent of the zirconium silicate beingof a particle size less than 100 mesh. The term mesh as used hereinrefers to the U. S. Sieve Series numbers as set forth,,fo r example, inthe data relating to Sieves of the Standard Screen Scale, page 2453 ofthe Handbook of Chemistry and Physics, 26th ed., 1942- 1943.

It has been found that while zirconium silicate alone has poorerelectrically insulating properties than magnesium oxide, in mixtures ofzirconium silicate and magnesium oxide of the above proportions, thesepoorer electrically insulating properties are offset by the betterheat-conducting properties of the zirconium silicate so that heaterelements containing such mixtures were completely satisfactory, bothfrom the heat conductivity and electrical insulating standpoints. .In

fact, the presence of a substantial quantity of quantity of silica,specifically fused silica in granular form and having a particle sizewithin the same ranges specified for the zirconium silicate. In general,the silica may be substituted for up to 60 per cent by weight of thezirconium silicate. In other words, the silica content of the totalrefractory mixture may be up to 40 per cent by weight.

As a specific example of how the present invention may be carried intoeffect, a powder mixture consisting of 7 parts zirconium silicate and 3parts crystalline fused magnesium oxide of the above-mentioned particlesizes was loaded into a sheathed unit employing the standard techniquedescribed, for example, in Oakley et a1. patent, 2,483,839. The loadedunit was prepared for a reduction in cross-sectional area by rolling inthe standard manner after first being sealed by means of washers crimpedinto the end of the tubular sheath. The units were thereafter rolled tocompact the powder and checked for elongation, degree of compaction, orhardness of the compacted material, reproducibility, and position ofrthehelical resistance elements. "It was found that all of the units loadedsatisfactorily and while they showed slightly greater elongation thancorresponding units filled only with pure magnesium oxide of the samedegree of compaction, they were nevertheless satisfactory andreproducible. In addition the helix was well centered within the tubularsheath and the comin the straight magnesium oxide pacted powder wasufliciently hard to retain the helix in such position during use of theheating unit. These results are to be compared with previous experiencesemploying pure zirconium silicate where after rolling, the helicalresistance element was poorly positioned within the tubular sheath andthe powder in the reduced unit was loose, permitting shifting of theelement during use of the heating unit.

In the insulating compositions of the present invention the zirconiumsilicate or mixture of zirconium silicate and silica comprises theprincipal portion. The magnesium oxide in the proportions employedpermits the proper compaction of the granular mixture which cannot beobtained with either pure zirconium silicate or mixtures of zirconiumsilicate and silica. It is essential that the magnesium oxide have agrain size larger than the particle sizes or average particle sizes ofthe zirconium silicate and silica. It is believed that the improvedcompacting characteristics result from the fact that the magnesium oxideis more easily crushed during the rolling or swaging operationsperformed on the sheathed unit and because of its cubic crystal habit,it breaks down in all directions with equal ease to fill in between theparticles of the zirconium silicate or mixtures of zirconium silicateand silica and effect a satisfactory compacting of the entire mixture.

Furthermore, by employing a mixture of different grain sizes there isobtained a material of a high bulk density and a resultant better heatconductivity. Furthermore, the larger grains of magnesium oxide breakdown more readily than smaller ones during the reducing operation withresultant better compaction of the mixtures.

It is to be noted that the mixtures of the present invention areparticularly characterized by the absence of any binder, such as clay orthe like, frequently employed for the purpose of bonding together poorlycompactible granular insulating materials ordinarily as a result of apartial sintering or fusing of the clay or similar binder. Such bindersare undesirable in the sheathed units with which the present inventionis concerned due to their poor electrical characteristics.

The advantages obtained by the substitution of some silica for part ofthe zirconium silicate in.- clude a decreased cost and a decrease inweight per unit volume. However, as the silica will not withstand ashigh an operating temperature as the zirconium silicate, the proportionsthereof should be limited, particularly in those instances where theheating unit will be operated at relatively high temperatures.

The primary function of the magnesium oxide is that of a binding actionwithout fusion. This binding action results solely from the compactingcharacteristics of the magnesium oxide and its ability to disintegrateduring the reducing operation and fill the voids between the zirconiumsilicate or zirconium silicate and silica particles.

What I claim as new and desire to secure by Letters Patent of the UnitedStates is:

1. A binder-free granular, refractory, heatconductive, embedding andelectrically insulating material for sheathing electrical resistanceelements comprising a mixture of (a) from about 20 to 40 per cent, byweight, granular fused magnesium oxide of a particle size such that atleast about 22 per cent is retained on a SO-mesh screen and all passesthrough a 40-mesh screen, and (b) from about 60 to by weight, of atleast one material of the class consisting of (1) zirconium silicate and(2) mixtures of zirconium siilcate and silica, said material (b) beingof a particle size less than 80 mesh with at least 80 per cent thereofbeing of a particle size less than 100 mesh.

2. An insulating material of claim 1 in which the material (b) iszirconium silicate.

3. The insulating material of claim 1 consisting of to 40 per centmagnesium oxide, to 70 per cent zirconium silicate and up to per centsilica.

4. The insulating material of claim 1 consisting of 30 per cent fusedmagnesium oxide, per cent fused zirconium silicate and 20 per centsilica.

5. A sheathed electrical heating element including an embedding materialcomprising a 0mm pressed, binder-free, granular mixture of 20 to 40 percent granular fused magnesia, balance at least one granular material ofthe group consisting of 1) zirconium silicate and (2) mixtures ofzirconium silicate and silica of a particle WILLIAM T. RAWLES.

References Gitetl in the file of this patent UNITED STATES PATENTSNumber Name Date 1,359,400 Lightfoot Nov. 16, 1920 1,952,119 ComstockMar. 27, 1934 OTHER REFERENCES Rees et al.: A Preliminary Investigationof the Magnesia-Zircon Series. Transactions of the English CeramicSociety, May 1930, pp. 309-316. Copy in Div. 3 (106-57).

